2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
5 Copyright (C) 2015-2019 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
7 Stockfish is free software: you can redistribute it and/or modify
8 it under the terms of the GNU General Public License as published by
9 the Free Software Foundation, either version 3 of the License, or
10 (at your option) any later version.
12 Stockfish is distributed in the hope that it will be useful,
13 but WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 GNU General Public License for more details.
17 You should have received a copy of the GNU General Public License
18 along with this program. If not, see <http://www.gnu.org/licenses/>.
24 #include <cstring> // For std::memset
38 #include "syzygy/tbprobe.h"
45 namespace Tablebases {
53 namespace TB = Tablebases;
57 using namespace Search;
61 // Different node types, used as a template parameter
62 enum NodeType { NonPV, PV };
64 // Razor and futility margins
65 constexpr int RazorMargin = 661;
66 Value futility_margin(Depth d, bool improving) {
67 return Value(198 * (d - improving));
70 // Reductions lookup table, initialized at startup
71 int Reductions[MAX_MOVES]; // [depth or moveNumber]
73 Depth reduction(bool i, Depth d, int mn) {
74 int r = Reductions[d] * Reductions[mn];
75 return (r + 520) / 1024 + (!i && r > 999);
78 constexpr int futility_move_count(bool improving, int depth) {
79 return (5 + depth * depth) * (1 + improving) / 2;
82 // History and stats update bonus, based on depth
83 int stat_bonus(Depth d) {
84 return d > 17 ? -8 : 22 * d * d + 151 * d - 140;
87 // Add a small random component to draw evaluations to avoid 3fold-blindness
88 Value value_draw(Thread* thisThread) {
89 return VALUE_DRAW + Value(2 * (thisThread->nodes & 1) - 1);
92 // Skill structure is used to implement strength limit
94 explicit Skill(int l) : level(l) {}
95 bool enabled() const { return level < 20; }
96 bool time_to_pick(Depth depth) const { return depth == 1 + level; }
97 Move pick_best(size_t multiPV);
100 Move best = MOVE_NONE;
103 // Breadcrumbs are used to mark nodes as being searched by a given thread
105 std::atomic<Thread*> thread;
106 std::atomic<Key> key;
108 std::array<Breadcrumb, 1024> breadcrumbs;
110 // ThreadHolding structure keeps track of which thread left breadcrumbs at the given
111 // node for potential reductions. A free node will be marked upon entering the moves
112 // loop by the constructor, and unmarked upon leaving that loop by the destructor.
113 struct ThreadHolding {
114 explicit ThreadHolding(Thread* thisThread, Key posKey, int ply) {
115 location = ply < 8 ? &breadcrumbs[posKey & (breadcrumbs.size() - 1)] : nullptr;
120 // See if another already marked this location, if not, mark it ourselves
121 Thread* tmp = (*location).thread.load(std::memory_order_relaxed);
124 (*location).thread.store(thisThread, std::memory_order_relaxed);
125 (*location).key.store(posKey, std::memory_order_relaxed);
128 else if ( tmp != thisThread
129 && (*location).key.load(std::memory_order_relaxed) == posKey)
135 if (owning) // Free the marked location
136 (*location).thread.store(nullptr, std::memory_order_relaxed);
139 bool marked() { return otherThread; }
142 Breadcrumb* location;
143 bool otherThread, owning;
146 template <NodeType NT>
147 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
149 template <NodeType NT>
150 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = 0);
152 Value value_to_tt(Value v, int ply);
153 Value value_from_tt(Value v, int ply);
154 void update_pv(Move* pv, Move move, Move* childPv);
155 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
156 void update_quiet_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietCount, int bonus);
157 void update_capture_stats(const Position& pos, Move move, Move* captures, int captureCount, int bonus);
159 // perft() is our utility to verify move generation. All the leaf nodes up
160 // to the given depth are generated and counted, and the sum is returned.
162 uint64_t perft(Position& pos, Depth depth) {
165 uint64_t cnt, nodes = 0;
166 const bool leaf = (depth == 2);
168 for (const auto& m : MoveList<LEGAL>(pos))
170 if (Root && depth <= 1)
175 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - 1);
180 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
188 /// Search::init() is called at startup to initialize various lookup tables
190 void Search::init() {
192 for (int i = 1; i < MAX_MOVES; ++i)
193 Reductions[i] = int((23.4 + std::log(Threads.size()) / 2) * std::log(i));
197 /// Search::clear() resets search state to its initial value
199 void Search::clear() {
201 Threads.main()->wait_for_search_finished();
203 Time.availableNodes = 0;
206 Tablebases::init(Options["SyzygyPath"]); // Free mapped files
210 /// MainThread::search() is started when the program receives the UCI 'go'
211 /// command. It searches from the root position and outputs the "bestmove".
213 void MainThread::search() {
217 nodes = perft<true>(rootPos, Limits.perft);
218 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
222 Color us = rootPos.side_to_move();
223 Time.init(Limits, us, rootPos.game_ply());
226 if (rootMoves.empty())
228 rootMoves.emplace_back(MOVE_NONE);
229 sync_cout << "info depth 0 score "
230 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
235 for (Thread* th : Threads)
237 th->bestMoveChanges = 0;
239 th->start_searching();
242 Thread::search(); // Let's start searching!
245 // When we reach the maximum depth, we can arrive here without a raise of
246 // Threads.stop. However, if we are pondering or in an infinite search,
247 // the UCI protocol states that we shouldn't print the best move before the
248 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
249 // until the GUI sends one of those commands.
251 while (!Threads.stop && (ponder || Limits.infinite))
252 {} // Busy wait for a stop or a ponder reset
254 // Stop the threads if not already stopped (also raise the stop if
255 // "ponderhit" just reset Threads.ponder).
258 // Wait until all threads have finished
259 for (Thread* th : Threads)
261 th->wait_for_search_finished();
263 // When playing in 'nodes as time' mode, subtract the searched nodes from
264 // the available ones before exiting.
266 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
268 Thread* bestThread = this;
270 // Check if there are threads with a better score than main thread
271 if ( Options["MultiPV"] == 1
273 && !(Skill(Options["Skill Level"]).enabled() || Options["UCI_LimitStrength"])
274 && rootMoves[0].pv[0] != MOVE_NONE)
276 std::map<Move, int64_t> votes;
277 Value minScore = this->rootMoves[0].score;
279 // Find out minimum score
280 for (Thread* th: Threads)
281 minScore = std::min(minScore, th->rootMoves[0].score);
283 // Vote according to score and depth, and select the best thread
284 for (Thread* th : Threads)
286 votes[th->rootMoves[0].pv[0]] +=
287 (th->rootMoves[0].score - minScore + 14) * int(th->completedDepth);
289 if (bestThread->rootMoves[0].score >= VALUE_MATE_IN_MAX_PLY)
291 // Make sure we pick the shortest mate
292 if (th->rootMoves[0].score > bestThread->rootMoves[0].score)
295 else if ( th->rootMoves[0].score >= VALUE_MATE_IN_MAX_PLY
296 || votes[th->rootMoves[0].pv[0]] > votes[bestThread->rootMoves[0].pv[0]])
301 previousScore = bestThread->rootMoves[0].score;
303 // Send again PV info if we have a new best thread
304 if (bestThread != this)
305 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
307 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
309 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
310 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
312 std::cout << sync_endl;
316 /// Thread::search() is the main iterative deepening loop. It calls search()
317 /// repeatedly with increasing depth until the allocated thinking time has been
318 /// consumed, the user stops the search, or the maximum search depth is reached.
320 void Thread::search() {
322 // To allow access to (ss-7) up to (ss+2), the stack must be oversized.
323 // The former is needed to allow update_continuation_histories(ss-1, ...),
324 // which accesses its argument at ss-6, also near the root.
325 // The latter is needed for statScores and killer initialization.
326 Stack stack[MAX_PLY+10], *ss = stack+7;
328 Value bestValue, alpha, beta, delta;
329 Move lastBestMove = MOVE_NONE;
330 Depth lastBestMoveDepth = 0;
331 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
332 double timeReduction = 1, totBestMoveChanges = 0;
333 Color us = rootPos.side_to_move();
335 std::memset(ss-7, 0, 10 * sizeof(Stack));
336 for (int i = 7; i > 0; i--)
337 (ss-i)->continuationHistory = &this->continuationHistory[0][0][NO_PIECE][0]; // Use as a sentinel
341 bestValue = delta = alpha = -VALUE_INFINITE;
342 beta = VALUE_INFINITE;
344 size_t multiPV = Options["MultiPV"];
346 // Pick integer skill levels, but non-deterministically round up or down
347 // such that the average integer skill corresponds to the input floating point one.
348 // UCI_Elo is converted to a suitable fractional skill level, using anchoring
349 // to CCRL Elo (goldfish 1.13 = 2000) and a fit through Ordo derived Elo
350 // for match (TC 60+0.6) results spanning a wide range of k values.
352 double floatLevel = Options["UCI_LimitStrength"] ?
353 clamp(std::pow((Options["UCI_Elo"] - 1346.6) / 143.4, 1 / 0.806), 0.0, 20.0) :
354 double(Options["Skill Level"]);
355 int intLevel = int(floatLevel) +
356 ((floatLevel - int(floatLevel)) * 1024 > rng.rand<unsigned>() % 1024 ? 1 : 0);
357 Skill skill(intLevel);
359 // When playing with strength handicap enable MultiPV search that we will
360 // use behind the scenes to retrieve a set of possible moves.
362 multiPV = std::max(multiPV, (size_t)4);
364 multiPV = std::min(multiPV, rootMoves.size());
366 int ct = int(Options["Contempt"]) * PawnValueEg / 100; // From centipawns
368 // In analysis mode, adjust contempt in accordance with user preference
369 if (Limits.infinite || Options["UCI_AnalyseMode"])
370 ct = Options["Analysis Contempt"] == "Off" ? 0
371 : Options["Analysis Contempt"] == "Both" ? ct
372 : Options["Analysis Contempt"] == "White" && us == BLACK ? -ct
373 : Options["Analysis Contempt"] == "Black" && us == WHITE ? -ct
376 // Evaluation score is from the white point of view
377 contempt = (us == WHITE ? make_score(ct, ct / 2)
378 : -make_score(ct, ct / 2));
380 // Iterative deepening loop until requested to stop or the target depth is reached
381 while ( ++rootDepth < MAX_PLY
383 && !(Limits.depth && mainThread && rootDepth > Limits.depth))
385 // Age out PV variability metric
387 totBestMoveChanges /= 2;
389 // Save the last iteration's scores before first PV line is searched and
390 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
391 for (RootMove& rm : rootMoves)
392 rm.previousScore = rm.score;
397 // MultiPV loop. We perform a full root search for each PV line
398 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
403 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
404 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
408 // Reset UCI info selDepth for each depth and each PV line
411 // Reset aspiration window starting size
414 Value previousScore = rootMoves[pvIdx].previousScore;
415 delta = Value(21 + abs(previousScore) / 128);
416 alpha = std::max(previousScore - delta,-VALUE_INFINITE);
417 beta = std::min(previousScore + delta, VALUE_INFINITE);
419 // Adjust contempt based on root move's previousScore (dynamic contempt)
420 int dct = ct + 86 * previousScore / (abs(previousScore) + 176);
422 contempt = (us == WHITE ? make_score(dct, dct / 2)
423 : -make_score(dct, dct / 2));
426 // Start with a small aspiration window and, in the case of a fail
427 // high/low, re-search with a bigger window until we don't fail
429 int failedHighCnt = 0;
432 Depth adjustedDepth = std::max(1, rootDepth - failedHighCnt);
433 bestValue = ::search<PV>(rootPos, ss, alpha, beta, adjustedDepth, false);
435 // Bring the best move to the front. It is critical that sorting
436 // is done with a stable algorithm because all the values but the
437 // first and eventually the new best one are set to -VALUE_INFINITE
438 // and we want to keep the same order for all the moves except the
439 // new PV that goes to the front. Note that in case of MultiPV
440 // search the already searched PV lines are preserved.
441 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
443 // If search has been stopped, we break immediately. Sorting is
444 // safe because RootMoves is still valid, although it refers to
445 // the previous iteration.
449 // When failing high/low give some update (without cluttering
450 // the UI) before a re-search.
453 && (bestValue <= alpha || bestValue >= beta)
454 && Time.elapsed() > 3000)
455 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
457 // In case of failing low/high increase aspiration window and
458 // re-search, otherwise exit the loop.
459 if (bestValue <= alpha)
461 beta = (alpha + beta) / 2;
462 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
466 mainThread->stopOnPonderhit = false;
468 else if (bestValue >= beta)
470 beta = std::min(bestValue + delta, VALUE_INFINITE);
475 ++rootMoves[pvIdx].bestMoveCount;
479 delta += delta / 4 + 5;
481 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
484 // Sort the PV lines searched so far and update the GUI
485 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
488 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
489 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
493 completedDepth = rootDepth;
495 if (rootMoves[0].pv[0] != lastBestMove) {
496 lastBestMove = rootMoves[0].pv[0];
497 lastBestMoveDepth = rootDepth;
500 // Have we found a "mate in x"?
502 && bestValue >= VALUE_MATE_IN_MAX_PLY
503 && VALUE_MATE - bestValue <= 2 * Limits.mate)
509 // If skill level is enabled and time is up, pick a sub-optimal best move
510 if (skill.enabled() && skill.time_to_pick(rootDepth))
511 skill.pick_best(multiPV);
513 // Do we have time for the next iteration? Can we stop searching now?
514 if ( Limits.use_time_management()
516 && !mainThread->stopOnPonderhit)
518 double fallingEval = (354 + 10 * (mainThread->previousScore - bestValue)) / 692.0;
519 fallingEval = clamp(fallingEval, 0.5, 1.5);
521 // If the bestMove is stable over several iterations, reduce time accordingly
522 timeReduction = lastBestMoveDepth + 9 < completedDepth ? 1.97 : 0.98;
523 double reduction = (1.36 + mainThread->previousTimeReduction) / (2.29 * timeReduction);
525 // Use part of the gained time from a previous stable move for the current move
526 for (Thread* th : Threads)
528 totBestMoveChanges += th->bestMoveChanges;
529 th->bestMoveChanges = 0;
531 double bestMoveInstability = 1 + totBestMoveChanges / Threads.size();
533 // Stop the search if we have only one legal move, or if available time elapsed
534 if ( rootMoves.size() == 1
535 || Time.elapsed() > Time.optimum() * fallingEval * reduction * bestMoveInstability)
537 // If we are allowed to ponder do not stop the search now but
538 // keep pondering until the GUI sends "ponderhit" or "stop".
539 if (mainThread->ponder)
540 mainThread->stopOnPonderhit = true;
550 mainThread->previousTimeReduction = timeReduction;
552 // If skill level is enabled, swap best PV line with the sub-optimal one
554 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
555 skill.best ? skill.best : skill.pick_best(multiPV)));
561 // search<>() is the main search function for both PV and non-PV nodes
563 template <NodeType NT>
564 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
566 constexpr bool PvNode = NT == PV;
567 const bool rootNode = PvNode && ss->ply == 0;
569 // Check if we have an upcoming move which draws by repetition, or
570 // if the opponent had an alternative move earlier to this position.
571 if ( pos.rule50_count() >= 3
572 && alpha < VALUE_DRAW
574 && pos.has_game_cycle(ss->ply))
576 alpha = value_draw(pos.this_thread());
581 // Dive into quiescence search when the depth reaches zero
583 return qsearch<NT>(pos, ss, alpha, beta);
585 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
586 assert(PvNode || (alpha == beta - 1));
587 assert(0 < depth && depth < MAX_PLY);
588 assert(!(PvNode && cutNode));
590 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
594 Move ttMove, move, excludedMove, bestMove;
595 Depth extension, newDepth;
596 Value bestValue, value, ttValue, eval, maxValue;
597 bool ttHit, ttPv, inCheck, givesCheck, improving, doLMR, priorCapture;
598 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, ttCapture;
600 int moveCount, captureCount, quietCount, singularLMR;
602 // Step 1. Initialize node
603 Thread* thisThread = pos.this_thread();
604 inCheck = pos.checkers();
605 priorCapture = pos.captured_piece();
606 Color us = pos.side_to_move();
607 moveCount = captureCount = quietCount = singularLMR = ss->moveCount = 0;
608 bestValue = -VALUE_INFINITE;
609 maxValue = VALUE_INFINITE;
611 // Check for the available remaining time
612 if (thisThread == Threads.main())
613 static_cast<MainThread*>(thisThread)->check_time();
615 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
616 if (PvNode && thisThread->selDepth < ss->ply + 1)
617 thisThread->selDepth = ss->ply + 1;
621 // Step 2. Check for aborted search and immediate draw
622 if ( Threads.stop.load(std::memory_order_relaxed)
623 || pos.is_draw(ss->ply)
624 || ss->ply >= MAX_PLY)
625 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos)
626 : value_draw(pos.this_thread());
628 // Step 3. Mate distance pruning. Even if we mate at the next move our score
629 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
630 // a shorter mate was found upward in the tree then there is no need to search
631 // because we will never beat the current alpha. Same logic but with reversed
632 // signs applies also in the opposite condition of being mated instead of giving
633 // mate. In this case return a fail-high score.
634 alpha = std::max(mated_in(ss->ply), alpha);
635 beta = std::min(mate_in(ss->ply+1), beta);
640 assert(0 <= ss->ply && ss->ply < MAX_PLY);
642 (ss+1)->ply = ss->ply + 1;
643 (ss+1)->excludedMove = bestMove = MOVE_NONE;
644 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
645 Square prevSq = to_sq((ss-1)->currentMove);
647 // Initialize statScore to zero for the grandchildren of the current position.
648 // So statScore is shared between all grandchildren and only the first grandchild
649 // starts with statScore = 0. Later grandchildren start with the last calculated
650 // statScore of the previous grandchild. This influences the reduction rules in
651 // LMR which are based on the statScore of parent position.
653 (ss+4)->statScore = 0;
655 (ss+2)->statScore = 0;
657 // Step 4. Transposition table lookup. We don't want the score of a partial
658 // search to overwrite a previous full search TT value, so we use a different
659 // position key in case of an excluded move.
660 excludedMove = ss->excludedMove;
661 posKey = pos.key() ^ Key(excludedMove << 16); // Isn't a very good hash
662 tte = TT.probe(posKey, ttHit);
663 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
664 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
665 : ttHit ? tte->move() : MOVE_NONE;
666 ttPv = PvNode || (ttHit && tte->is_pv());
668 // At non-PV nodes we check for an early TT cutoff
671 && tte->depth() >= depth
672 && ttValue != VALUE_NONE // Possible in case of TT access race
673 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
674 : (tte->bound() & BOUND_UPPER)))
676 // If ttMove is quiet, update move sorting heuristics on TT hit
681 if (!pos.capture_or_promotion(ttMove))
682 update_quiet_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
684 // Extra penalty for early quiet moves of the previous ply
685 if ((ss-1)->moveCount <= 2 && !priorCapture)
686 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + 1));
688 // Penalty for a quiet ttMove that fails low
689 else if (!pos.capture_or_promotion(ttMove))
691 int penalty = -stat_bonus(depth);
692 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
693 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
699 // Step 5. Tablebases probe
700 if (!rootNode && TB::Cardinality)
702 int piecesCount = pos.count<ALL_PIECES>();
704 if ( piecesCount <= TB::Cardinality
705 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
706 && pos.rule50_count() == 0
707 && !pos.can_castle(ANY_CASTLING))
710 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
712 // Force check of time on the next occasion
713 if (thisThread == Threads.main())
714 static_cast<MainThread*>(thisThread)->callsCnt = 0;
716 if (err != TB::ProbeState::FAIL)
718 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
720 int drawScore = TB::UseRule50 ? 1 : 0;
722 value = wdl < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
723 : wdl > drawScore ? VALUE_MATE - MAX_PLY - ss->ply - 1
724 : VALUE_DRAW + 2 * wdl * drawScore;
726 Bound b = wdl < -drawScore ? BOUND_UPPER
727 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
729 if ( b == BOUND_EXACT
730 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
732 tte->save(posKey, value_to_tt(value, ss->ply), ttPv, b,
733 std::min(MAX_PLY - 1, depth + 6),
734 MOVE_NONE, VALUE_NONE);
741 if (b == BOUND_LOWER)
742 bestValue = value, alpha = std::max(alpha, bestValue);
750 // Step 6. Static evaluation of the position
753 ss->staticEval = eval = VALUE_NONE;
755 goto moves_loop; // Skip early pruning when in check
759 // Never assume anything about values stored in TT
760 ss->staticEval = eval = tte->eval();
761 if (eval == VALUE_NONE)
762 ss->staticEval = eval = evaluate(pos);
764 if (eval == VALUE_DRAW)
765 eval = value_draw(thisThread);
767 // Can ttValue be used as a better position evaluation?
768 if ( ttValue != VALUE_NONE
769 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
774 if ((ss-1)->currentMove != MOVE_NULL)
776 int bonus = -(ss-1)->statScore / 512;
778 ss->staticEval = eval = evaluate(pos) + bonus;
781 ss->staticEval = eval = -(ss-1)->staticEval + 2 * Eval::Tempo;
783 tte->save(posKey, VALUE_NONE, ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
786 // Step 7. Razoring (~2 Elo)
787 if ( !rootNode // The required rootNode PV handling is not available in qsearch
789 && eval <= alpha - RazorMargin)
790 return qsearch<NT>(pos, ss, alpha, beta);
792 improving = ss->staticEval >= (ss-2)->staticEval
793 || (ss-2)->staticEval == VALUE_NONE;
795 // Step 8. Futility pruning: child node (~30 Elo)
798 && eval - futility_margin(depth, improving) >= beta
799 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
802 // Step 9. Null move search with verification search (~40 Elo)
804 && (ss-1)->currentMove != MOVE_NULL
805 && (ss-1)->statScore < 22661
807 && eval >= ss->staticEval
808 && ss->staticEval >= beta - 33 * depth + 299 - improving * 30
810 && pos.non_pawn_material(us)
811 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
813 assert(eval - beta >= 0);
815 // Null move dynamic reduction based on depth and value
816 Depth R = (835 + 70 * depth) / 256 + std::min(int(eval - beta) / 185, 3);
818 ss->currentMove = MOVE_NULL;
819 ss->continuationHistory = &thisThread->continuationHistory[0][0][NO_PIECE][0];
821 pos.do_null_move(st);
823 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
825 pos.undo_null_move();
827 if (nullValue >= beta)
829 // Do not return unproven mate scores
830 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
833 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 13))
836 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
838 // Do verification search at high depths, with null move pruning disabled
839 // for us, until ply exceeds nmpMinPly.
840 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
841 thisThread->nmpColor = us;
843 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
845 thisThread->nmpMinPly = 0;
852 // Step 10. ProbCut (~10 Elo)
853 // If we have a good enough capture and a reduced search returns a value
854 // much above beta, we can (almost) safely prune the previous move.
857 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
859 Value raisedBeta = std::min(beta + 191 - 46 * improving, VALUE_INFINITE);
860 MovePicker mp(pos, ttMove, raisedBeta - ss->staticEval, &thisThread->captureHistory);
861 int probCutCount = 0;
863 while ( (move = mp.next_move()) != MOVE_NONE
864 && probCutCount < 2 + 2 * cutNode)
865 if (move != excludedMove && pos.legal(move))
869 ss->currentMove = move;
870 ss->continuationHistory = &thisThread->continuationHistory[inCheck][priorCapture][pos.moved_piece(move)][to_sq(move)];
874 pos.do_move(move, st);
876 // Perform a preliminary qsearch to verify that the move holds
877 value = -qsearch<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1);
879 // If the qsearch held, perform the regular search
880 if (value >= raisedBeta)
881 value = -search<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1, depth - 4, !cutNode);
885 if (value >= raisedBeta)
890 // Step 11. Internal iterative deepening (~2 Elo)
891 if (depth >= 7 && !ttMove)
893 search<NT>(pos, ss, alpha, beta, depth - 7, cutNode);
895 tte = TT.probe(posKey, ttHit);
896 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
897 ttMove = ttHit ? tte->move() : MOVE_NONE;
900 moves_loop: // When in check, search starts from here
902 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
903 nullptr, (ss-4)->continuationHistory,
904 nullptr, (ss-6)->continuationHistory };
906 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
908 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
909 &thisThread->captureHistory,
914 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
915 moveCountPruning = false;
916 ttCapture = ttMove && pos.capture_or_promotion(ttMove);
918 // Mark this node as being searched
919 ThreadHolding th(thisThread, posKey, ss->ply);
921 // Step 12. Loop through all pseudo-legal moves until no moves remain
922 // or a beta cutoff occurs.
923 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
927 if (move == excludedMove)
930 // At root obey the "searchmoves" option and skip moves not listed in Root
931 // Move List. As a consequence any illegal move is also skipped. In MultiPV
932 // mode we also skip PV moves which have been already searched and those
933 // of lower "TB rank" if we are in a TB root position.
934 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
935 thisThread->rootMoves.begin() + thisThread->pvLast, move))
938 ss->moveCount = ++moveCount;
940 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
941 sync_cout << "info depth " << depth
942 << " currmove " << UCI::move(move, pos.is_chess960())
943 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
945 (ss+1)->pv = nullptr;
948 captureOrPromotion = pos.capture_or_promotion(move);
949 movedPiece = pos.moved_piece(move);
950 givesCheck = pos.gives_check(move);
952 // Step 13. Extensions (~70 Elo)
954 // Singular extension search (~60 Elo). If all moves but one fail low on a
955 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
956 // then that move is singular and should be extended. To verify this we do
957 // a reduced search on all the other moves but the ttMove and if the
958 // result is lower than ttValue minus a margin then we will extend the ttMove.
962 && !excludedMove // Avoid recursive singular search
963 /* && ttValue != VALUE_NONE Already implicit in the next condition */
964 && abs(ttValue) < VALUE_KNOWN_WIN
965 && (tte->bound() & BOUND_LOWER)
966 && tte->depth() >= depth - 3
969 Value singularBeta = ttValue - 2 * depth;
970 Depth halfDepth = depth / 2;
971 ss->excludedMove = move;
972 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, halfDepth, cutNode);
973 ss->excludedMove = MOVE_NONE;
975 if (value < singularBeta)
980 if (value < singularBeta - std::min(4 * depth, 36))
985 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
986 // search without the ttMove. So we assume this expected Cut-node is not singular,
987 // that multiple moves fail high, and we can prune the whole subtree by returning
989 else if ( eval >= beta
990 && singularBeta >= beta)
994 // Check extension (~2 Elo)
996 && (pos.is_discovery_check_on_king(~us, move) || pos.see_ge(move)))
1001 && pos.rule50_count() > 18
1003 && ++thisThread->shuffleExts < thisThread->nodes.load(std::memory_order_relaxed) / 4) // To avoid too many extensions
1006 // Passed pawn extension
1007 else if ( move == ss->killers[0]
1008 && pos.advanced_pawn_push(move)
1009 && pos.pawn_passed(us, to_sq(move)))
1012 // Castling extension
1013 if (type_of(move) == CASTLING)
1016 // Calculate new depth for this move
1017 newDepth = depth - 1 + extension;
1019 // Step 14. Pruning at shallow depth (~170 Elo)
1021 && pos.non_pawn_material(us)
1022 && bestValue > VALUE_MATED_IN_MAX_PLY)
1024 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold
1025 moveCountPruning = moveCount >= futility_move_count(improving, depth);
1027 if ( !captureOrPromotion
1029 && (!pos.advanced_pawn_push(move) || pos.non_pawn_material(~us) > BishopValueMg))
1031 // Move count based pruning
1032 if (moveCountPruning)
1035 // Reduced depth of the next LMR search
1036 int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount), 0);
1038 // Countermoves based pruning (~20 Elo)
1039 if ( lmrDepth < 4 + ((ss-1)->statScore > 0 || (ss-1)->moveCount == 1)
1040 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
1041 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
1044 // Futility pruning: parent node (~2 Elo)
1047 && ss->staticEval + 250 + 211 * lmrDepth <= alpha)
1050 // Prune moves with negative SEE (~10 Elo)
1051 if (!pos.see_ge(move, Value(-(31 - std::min(lmrDepth, 18)) * lmrDepth * lmrDepth)))
1054 else if ( !(givesCheck && extension)
1055 && !pos.see_ge(move, Value(-199) * depth)) // (~20 Elo)
1059 // Speculative prefetch as early as possible
1060 prefetch(TT.first_entry(pos.key_after(move)));
1062 // Check for legality just before making the move
1063 if (!rootNode && !pos.legal(move))
1065 ss->moveCount = --moveCount;
1069 // Update the current move (this must be done after singular extension search)
1070 ss->currentMove = move;
1071 ss->continuationHistory = &thisThread->continuationHistory[inCheck][priorCapture][movedPiece][to_sq(move)];
1073 // Step 15. Make the move
1074 pos.do_move(move, st, givesCheck);
1076 // Step 16. Reduced depth search (LMR). If the move fails high it will be
1077 // re-searched at full depth.
1079 && moveCount > 1 + 2 * rootNode
1080 && (!rootNode || thisThread->best_move_count(move) == 0)
1081 && ( !captureOrPromotion
1083 || ss->staticEval + PieceValue[EG][pos.captured_piece()] <= alpha
1086 Depth r = reduction(improving, depth, moveCount);
1088 // Reduction if other threads are searching this position.
1092 // Decrease reduction if position is or has been on the PV
1096 // Decrease reduction if opponent's move count is high (~10 Elo)
1097 if ((ss-1)->moveCount > 15)
1100 // Decrease reduction if ttMove has been singularly extended
1103 if (!captureOrPromotion)
1105 // Increase reduction if ttMove is a capture (~0 Elo)
1109 // Increase reduction for cut nodes (~5 Elo)
1113 // Decrease reduction for moves that escape a capture. Filter out
1114 // castling moves, because they are coded as "king captures rook" and
1115 // hence break make_move(). (~5 Elo)
1116 else if ( type_of(move) == NORMAL
1117 && !pos.see_ge(reverse_move(move)))
1120 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1121 + (*contHist[0])[movedPiece][to_sq(move)]
1122 + (*contHist[1])[movedPiece][to_sq(move)]
1123 + (*contHist[3])[movedPiece][to_sq(move)]
1126 // Reset statScore to zero if negative and most stats shows >= 0
1127 if ( ss->statScore < 0
1128 && (*contHist[0])[movedPiece][to_sq(move)] >= 0
1129 && (*contHist[1])[movedPiece][to_sq(move)] >= 0
1130 && thisThread->mainHistory[us][from_to(move)] >= 0)
1133 // Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
1134 if (ss->statScore >= -99 && (ss-1)->statScore < -116)
1137 else if ((ss-1)->statScore >= -117 && ss->statScore < -144)
1140 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1141 r -= ss->statScore / 16384;
1144 Depth d = clamp(newDepth - r, 1, newDepth);
1146 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1148 doFullDepthSearch = (value > alpha && d != newDepth), doLMR = true;
1151 doFullDepthSearch = !PvNode || moveCount > 1, doLMR = false;
1153 // Step 17. Full depth search when LMR is skipped or fails high
1154 if (doFullDepthSearch)
1156 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1158 if (doLMR && !captureOrPromotion)
1160 int bonus = value > alpha ? stat_bonus(newDepth)
1161 : -stat_bonus(newDepth);
1163 if (move == ss->killers[0])
1166 update_continuation_histories(ss, movedPiece, to_sq(move), bonus);
1170 // For PV nodes only, do a full PV search on the first move or after a fail
1171 // high (in the latter case search only if value < beta), otherwise let the
1172 // parent node fail low with value <= alpha and try another move.
1173 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1176 (ss+1)->pv[0] = MOVE_NONE;
1178 value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1181 // Step 18. Undo move
1182 pos.undo_move(move);
1184 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1186 // Step 19. Check for a new best move
1187 // Finished searching the move. If a stop occurred, the return value of
1188 // the search cannot be trusted, and we return immediately without
1189 // updating best move, PV and TT.
1190 if (Threads.stop.load(std::memory_order_relaxed))
1195 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1196 thisThread->rootMoves.end(), move);
1198 // PV move or new best move?
1199 if (moveCount == 1 || value > alpha)
1202 rm.selDepth = thisThread->selDepth;
1207 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1208 rm.pv.push_back(*m);
1210 // We record how often the best move has been changed in each
1211 // iteration. This information is used for time management: When
1212 // the best move changes frequently, we allocate some more time.
1214 ++thisThread->bestMoveChanges;
1217 // All other moves but the PV are set to the lowest value: this
1218 // is not a problem when sorting because the sort is stable and the
1219 // move position in the list is preserved - just the PV is pushed up.
1220 rm.score = -VALUE_INFINITE;
1223 if (value > bestValue)
1231 if (PvNode && !rootNode) // Update pv even in fail-high case
1232 update_pv(ss->pv, move, (ss+1)->pv);
1234 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1238 assert(value >= beta); // Fail high
1245 if (move != bestMove)
1247 if (captureOrPromotion && captureCount < 32)
1248 capturesSearched[captureCount++] = move;
1250 else if (!captureOrPromotion && quietCount < 64)
1251 quietsSearched[quietCount++] = move;
1255 // The following condition would detect a stop only after move loop has been
1256 // completed. But in this case bestValue is valid because we have fully
1257 // searched our subtree, and we can anyhow save the result in TT.
1263 // Step 20. Check for mate and stalemate
1264 // All legal moves have been searched and if there are no legal moves, it
1265 // must be a mate or a stalemate. If we are in a singular extension search then
1266 // return a fail low score.
1268 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1271 bestValue = excludedMove ? alpha
1272 : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1275 // Quiet best move: update move sorting heuristics
1276 if (!pos.capture_or_promotion(bestMove))
1277 update_quiet_stats(pos, ss, bestMove, quietsSearched, quietCount,
1278 stat_bonus(depth + (bestValue > beta + PawnValueMg)));
1280 update_capture_stats(pos, bestMove, capturesSearched, captureCount, stat_bonus(depth + 1));
1282 // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
1283 if ( ((ss-1)->moveCount == 1 || ((ss-1)->currentMove == (ss-1)->killers[0]))
1285 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + 1));
1288 // Bonus for prior countermove that caused the fail low
1289 else if ( (depth >= 3 || PvNode)
1291 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1294 bestValue = std::min(bestValue, maxValue);
1297 tte->save(posKey, value_to_tt(bestValue, ss->ply), ttPv,
1298 bestValue >= beta ? BOUND_LOWER :
1299 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1300 depth, bestMove, ss->staticEval);
1302 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1308 // qsearch() is the quiescence search function, which is called by the main search
1309 // function with zero depth, or recursively with further decreasing depth per call.
1310 template <NodeType NT>
1311 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1313 constexpr bool PvNode = NT == PV;
1315 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1316 assert(PvNode || (alpha == beta - 1));
1323 Move ttMove, move, bestMove;
1325 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1326 bool ttHit, pvHit, inCheck, givesCheck, evasionPrunable, priorCapture;
1331 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1333 ss->pv[0] = MOVE_NONE;
1336 Thread* thisThread = pos.this_thread();
1337 (ss+1)->ply = ss->ply + 1;
1338 bestMove = MOVE_NONE;
1339 inCheck = pos.checkers();
1340 priorCapture = pos.captured_piece();
1343 // Check for an immediate draw or maximum ply reached
1344 if ( pos.is_draw(ss->ply)
1345 || ss->ply >= MAX_PLY)
1346 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
1348 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1350 // Decide whether or not to include checks: this fixes also the type of
1351 // TT entry depth that we are going to use. Note that in qsearch we use
1352 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1353 ttDepth = inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1354 : DEPTH_QS_NO_CHECKS;
1355 // Transposition table lookup
1357 tte = TT.probe(posKey, ttHit);
1358 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1359 ttMove = ttHit ? tte->move() : MOVE_NONE;
1360 pvHit = ttHit && tte->is_pv();
1364 && tte->depth() >= ttDepth
1365 && ttValue != VALUE_NONE // Only in case of TT access race
1366 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1367 : (tte->bound() & BOUND_UPPER)))
1370 // Evaluate the position statically
1373 ss->staticEval = VALUE_NONE;
1374 bestValue = futilityBase = -VALUE_INFINITE;
1380 // Never assume anything about values stored in TT
1381 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1382 ss->staticEval = bestValue = evaluate(pos);
1384 // Can ttValue be used as a better position evaluation?
1385 if ( ttValue != VALUE_NONE
1386 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1387 bestValue = ttValue;
1390 ss->staticEval = bestValue =
1391 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1392 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1394 // Stand pat. Return immediately if static value is at least beta
1395 if (bestValue >= beta)
1398 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit, BOUND_LOWER,
1399 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1404 if (PvNode && bestValue > alpha)
1407 futilityBase = bestValue + 153;
1410 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1411 nullptr, (ss-4)->continuationHistory,
1412 nullptr, (ss-6)->continuationHistory };
1414 // Initialize a MovePicker object for the current position, and prepare
1415 // to search the moves. Because the depth is <= 0 here, only captures,
1416 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1418 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1419 &thisThread->captureHistory,
1421 to_sq((ss-1)->currentMove));
1423 // Loop through the moves until no moves remain or a beta cutoff occurs
1424 while ((move = mp.next_move()) != MOVE_NONE)
1426 assert(is_ok(move));
1428 givesCheck = pos.gives_check(move);
1435 && futilityBase > -VALUE_KNOWN_WIN
1436 && !pos.advanced_pawn_push(move))
1438 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1440 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1442 if (futilityValue <= alpha)
1444 bestValue = std::max(bestValue, futilityValue);
1448 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1450 bestValue = std::max(bestValue, futilityBase);
1455 // Detect non-capture evasions that are candidates to be pruned
1456 evasionPrunable = inCheck
1457 && (depth != 0 || moveCount > 2)
1458 && bestValue > VALUE_MATED_IN_MAX_PLY
1459 && !pos.capture(move);
1461 // Don't search moves with negative SEE values
1462 if ( (!inCheck || evasionPrunable)
1463 && (!givesCheck || !(pos.blockers_for_king(~pos.side_to_move()) & from_sq(move)))
1464 && !pos.see_ge(move))
1467 // Speculative prefetch as early as possible
1468 prefetch(TT.first_entry(pos.key_after(move)));
1470 // Check for legality just before making the move
1471 if (!pos.legal(move))
1477 ss->currentMove = move;
1478 ss->continuationHistory = &thisThread->continuationHistory[inCheck][priorCapture][pos.moved_piece(move)][to_sq(move)];
1480 // Make and search the move
1481 pos.do_move(move, st, givesCheck);
1482 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - 1);
1483 pos.undo_move(move);
1485 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1487 // Check for a new best move
1488 if (value > bestValue)
1496 if (PvNode) // Update pv even in fail-high case
1497 update_pv(ss->pv, move, (ss+1)->pv);
1499 if (PvNode && value < beta) // Update alpha here!
1507 // All legal moves have been searched. A special case: If we're in check
1508 // and no legal moves were found, it is checkmate.
1509 if (inCheck && bestValue == -VALUE_INFINITE)
1510 return mated_in(ss->ply); // Plies to mate from the root
1512 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1513 bestValue >= beta ? BOUND_LOWER :
1514 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1515 ttDepth, bestMove, ss->staticEval);
1517 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1523 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1524 // "plies to mate from the current position". Non-mate scores are unchanged.
1525 // The function is called before storing a value in the transposition table.
1527 Value value_to_tt(Value v, int ply) {
1529 assert(v != VALUE_NONE);
1531 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1532 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1536 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1537 // from the transposition table (which refers to the plies to mate/be mated
1538 // from current position) to "plies to mate/be mated from the root".
1540 Value value_from_tt(Value v, int ply) {
1542 return v == VALUE_NONE ? VALUE_NONE
1543 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1544 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1548 // update_pv() adds current move and appends child pv[]
1550 void update_pv(Move* pv, Move move, Move* childPv) {
1552 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1558 // update_continuation_histories() updates histories of the move pairs formed
1559 // by moves at ply -1, -2, and -4 with current move.
1561 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1563 for (int i : {1, 2, 4, 6})
1564 if (is_ok((ss-i)->currentMove))
1565 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1569 // update_capture_stats() updates move sorting heuristics when a new capture best move is found
1571 void update_capture_stats(const Position& pos, Move move,
1572 Move* captures, int captureCount, int bonus) {
1574 CapturePieceToHistory& captureHistory = pos.this_thread()->captureHistory;
1575 Piece moved_piece = pos.moved_piece(move);
1576 PieceType captured = type_of(pos.piece_on(to_sq(move)));
1578 if (pos.capture_or_promotion(move))
1579 captureHistory[moved_piece][to_sq(move)][captured] << bonus;
1581 // Decrease all the other played capture moves
1582 for (int i = 0; i < captureCount; ++i)
1584 moved_piece = pos.moved_piece(captures[i]);
1585 captured = type_of(pos.piece_on(to_sq(captures[i])));
1586 captureHistory[moved_piece][to_sq(captures[i])][captured] << -bonus;
1591 // update_quiet_stats() updates move sorting heuristics when a new quiet best move is found
1593 void update_quiet_stats(const Position& pos, Stack* ss, Move move,
1594 Move* quiets, int quietCount, int bonus) {
1596 if (ss->killers[0] != move)
1598 ss->killers[1] = ss->killers[0];
1599 ss->killers[0] = move;
1602 Color us = pos.side_to_move();
1603 Thread* thisThread = pos.this_thread();
1604 thisThread->mainHistory[us][from_to(move)] << bonus;
1605 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1607 if (type_of(pos.moved_piece(move)) != PAWN)
1608 thisThread->mainHistory[us][from_to(reverse_move(move))] << -bonus;
1610 if (is_ok((ss-1)->currentMove))
1612 Square prevSq = to_sq((ss-1)->currentMove);
1613 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1616 // Decrease all the other played quiet moves
1617 for (int i = 0; i < quietCount; ++i)
1619 thisThread->mainHistory[us][from_to(quiets[i])] << -bonus;
1620 update_continuation_histories(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1624 // When playing with strength handicap, choose best move among a set of RootMoves
1625 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1627 Move Skill::pick_best(size_t multiPV) {
1629 const RootMoves& rootMoves = Threads.main()->rootMoves;
1630 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1632 // RootMoves are already sorted by score in descending order
1633 Value topScore = rootMoves[0].score;
1634 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1635 int weakness = 120 - 2 * level;
1636 int maxScore = -VALUE_INFINITE;
1638 // Choose best move. For each move score we add two terms, both dependent on
1639 // weakness. One is deterministic and bigger for weaker levels, and one is
1640 // random. Then we choose the move with the resulting highest score.
1641 for (size_t i = 0; i < multiPV; ++i)
1643 // This is our magic formula
1644 int push = ( weakness * int(topScore - rootMoves[i].score)
1645 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1647 if (rootMoves[i].score + push >= maxScore)
1649 maxScore = rootMoves[i].score + push;
1650 best = rootMoves[i].pv[0];
1659 /// MainThread::check_time() is used to print debug info and, more importantly,
1660 /// to detect when we are out of available time and thus stop the search.
1662 void MainThread::check_time() {
1667 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1668 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1670 static TimePoint lastInfoTime = now();
1672 TimePoint elapsed = Time.elapsed();
1673 TimePoint tick = Limits.startTime + elapsed;
1675 if (tick - lastInfoTime >= 1000)
1677 lastInfoTime = tick;
1681 // We should not stop pondering until told so by the GUI
1685 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1686 || (Limits.movetime && elapsed >= Limits.movetime)
1687 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1688 Threads.stop = true;
1692 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1693 /// that all (if any) unsearched PV lines are sent using a previous search score.
1695 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1697 std::stringstream ss;
1698 TimePoint elapsed = Time.elapsed() + 1;
1699 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1700 size_t pvIdx = pos.this_thread()->pvIdx;
1701 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1702 uint64_t nodesSearched = Threads.nodes_searched();
1703 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1705 for (size_t i = 0; i < multiPV; ++i)
1707 bool updated = (i <= pvIdx && rootMoves[i].score != -VALUE_INFINITE);
1709 if (depth == 1 && !updated)
1712 Depth d = updated ? depth : depth - 1;
1713 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1715 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1716 v = tb ? rootMoves[i].tbScore : v;
1718 if (ss.rdbuf()->in_avail()) // Not at first line
1723 << " seldepth " << rootMoves[i].selDepth
1724 << " multipv " << i + 1
1725 << " score " << UCI::value(v);
1727 if (!tb && i == pvIdx)
1728 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1730 ss << " nodes " << nodesSearched
1731 << " nps " << nodesSearched * 1000 / elapsed;
1733 if (elapsed > 1000) // Earlier makes little sense
1734 ss << " hashfull " << TT.hashfull();
1736 ss << " tbhits " << tbHits
1737 << " time " << elapsed
1740 for (Move m : rootMoves[i].pv)
1741 ss << " " << UCI::move(m, pos.is_chess960());
1748 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1749 /// before exiting the search, for instance, in case we stop the search during a
1750 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1751 /// otherwise in case of 'ponder on' we have nothing to think on.
1753 bool RootMove::extract_ponder_from_tt(Position& pos) {
1758 assert(pv.size() == 1);
1760 if (pv[0] == MOVE_NONE)
1763 pos.do_move(pv[0], st);
1764 TTEntry* tte = TT.probe(pos.key(), ttHit);
1768 Move m = tte->move(); // Local copy to be SMP safe
1769 if (MoveList<LEGAL>(pos).contains(m))
1773 pos.undo_move(pv[0]);
1774 return pv.size() > 1;
1777 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1780 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1781 ProbeDepth = int(Options["SyzygyProbeDepth"]);
1782 Cardinality = int(Options["SyzygyProbeLimit"]);
1783 bool dtz_available = true;
1785 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1786 // ProbeDepth == DEPTH_ZERO
1787 if (Cardinality > MaxCardinality)
1789 Cardinality = MaxCardinality;
1793 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1795 // Rank moves using DTZ tables
1796 RootInTB = root_probe(pos, rootMoves);
1800 // DTZ tables are missing; try to rank moves using WDL tables
1801 dtz_available = false;
1802 RootInTB = root_probe_wdl(pos, rootMoves);
1808 // Sort moves according to TB rank
1809 std::sort(rootMoves.begin(), rootMoves.end(),
1810 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1812 // Probe during search only if DTZ is not available and we are winning
1813 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1818 // Clean up if root_probe() and root_probe_wdl() have failed
1819 for (auto& m : rootMoves)